In my many years of FFXI I've come to understand that the fish's HP is client-sided information only, where the server has no monitor on it until it's 0%, wherein it can accept the user's input to reel in the fish/item/monster/whatever.
Because of this many bots simply just alter the fish's HP value to 0, without actually fighting the fish. This straight 100% -> 0% should be a huge indicator that a bot is being used. So, SE, to help cut back on abuse, making the HP server-sided is a step in the right direction. All of these bots can, presumably, simply fight the fish after the fix. But I'm sure it will throw many of them off.
Thanks for reading.